This invention relates to a disc of a disc brake assembly of the type which is well known for example for industrial and automotive applications.
In such an assembly the disc, or rotor as it is sometimes known, is rotatable with a member to be braked such as a wheel. A pair of opposed relatively axially movable brake pads carried in a brake caliper are arranged one on either side of the disc so as to exert braking pressure on the rotating disc, generated typically by a hydraulic piston and cylinder arrangement, when it is desired to brake the said member. The wear surfaces of the disc which are acted on by the opposed brake pads comprise annular planar surfaces, one on each side of the disc, located radially intermediate the radially inner and outer edges of the disc.
The discs of such disc brake assemblies are typically formed of a ferrous alloy having the properties necessary to withstand repeated applications of braking pressure by the brake pads. Such properties include good wear resistance and good thermal conductivity, and conventional brake discs have been formed of cast iron to provide these properties. For automotive applications, especially for racing motorcycles or racing cars, there is a requirement to reduce weight wherever possible and hence it has been proposed to utilise a brake disc formed of a lighter weight material than ferrous alloys.
For example, it has been proposed to utilise, aluminum alloys instead of ferrous alloys because a brake disc formed of an aluminum alloy would be of much less weight than a correspondingly sized ferrous alloy disc. An aluminum alloy disc would also exhibit the required good thermal conductivity but a major disadvantage is that the melting point of an aluminum alloy is much lower than that of a ferrous alloy, also the wear resistance of an aluminum alloy is much less than that of e.g. cast iron. Repeated applications of braking pressure to an aluminum alloy disc would generate sufficient heat to soften the aluminum alloy and destroy the disc.
To overcome the problems inherent with aluminum alloys one proposal has been to form a brake disc from a metal matrix composite comprising discrete particles, fibres or whiskers of a ceramic material distributed throughout an aluminum alloy matrix. This proposal, although imparting the necessary high wear resistance properties to the brake disc, is not satisfactory because the high temperatures generated by the type of severe, repeated braking conditions encountered in racing, still results in softening of the aluminum alloy matrix. If the matrix is softened sufficiently it will be expelled, together with the ceramic particles, fibres or whiskers contained therein, from those wear areas of the disc which are acted on by the brake pads and the disc will effectively be destroyed.
Another proposal to overcome the problems inherent with aluminum alloy brake discs has been to coat at least the wear surfaces of the disc with a wear resistant and temperature resistant material. This proposal would increase the production cost of each brake disc and is not satisfactory in that the high temperatures generated during severe braking may still cause the aluminum alloy body of the disc to soften with consequential risk of sudden total failure.
It is an object of the present invention to provide a new and improved brake disc which will overcome the problems associated hitherto with aluminum alloy discs. It is also an object of the invention to provide a new and improved method for the production of such a brake disc.